Zoom lens system in which the rear or image side lens units switch out of and into an optical path are known. For example, U.S. Pat. No. 4,871,238, which issued to Sato et al., on Oct. 3, 1989, discloses a photographic optical device including a master lens unit having a positive refractive power; a first auxiliary lens unit having a positive refractive power capable of being placed on and off the optical path; and a second auxiliary lens unit having a negative refiactive power capable of being placed on and off the optical path. The first auxiliary lens unit is attached to an image-plane side of the master lens unit to form a photographic unit giving a low ratio of magnification. The second auxiliary lens unit is attached to an image-plane side of the master lens unit to form another photographic unit giving a high ratio of magnification. The first and second auxiliary lens units are placed outside of the optical path to form another photographic unit giving a middle ratio of magnification. The zoom lens system disclosed in U.S. Pat. No. 4,871,238 is disadvantaged in that each lens unit requires many individual lens components which increases manufacturing costs associated with building the zoom lens system and increases the complexity of the design of the zoom lens system. Additionally, the zoom lens system disclosed in U.S. Pat. No. 4,871,238 includes a wide angle, mid- range, and telephoto format (lens position) which increases the cost and complexity associated with the lens driving mechanism.
Lens systems having a reduced number of individual lens components are also known. For example, U.S. Pat. No. 5,677,798, which issued to Hirano et al., on Oct. 14, 1997, discloses an image forming lens system including a positive front lens group, a diaphragm, and a rear lens group, in order from an object to be imaged. The front lens group is made of optical glass. The rear lens group is made of a single meniscus plastic lens having opposed aspheric lens surfaces with a concave surface adjacent to the diaphragm. Additionally, U.S. Pat. No. 5,067,803, which issued to Ohno, on Nov. 26, 1991, discloses a photographic wide angle lens which is composed of, from an object side thereof, a first lens of positive meniscus having a convex surface on the object side, and a second lens of positive meniscus having a concave surface on the object side. The wide angle lens has an aspheric surface at least on one of the four lens surfaces provided by the first and second lenses. While the lens systems disclosed in U.S. Pat. Nos. 5,677,798 and 5,067,803 have a reduced number of individual lens elements, each lens system is disadvantaged in that the ratio of magnification of the lens system can not be changed.
Recent surveys of picture taking consumers using a zoom lens indicated that over 90% of all photographs are shot in either a wide angle format or a telephoto format. As such, there is a need for a variable focal length lens system having a reduced number of individual lens components capable of switching between a wide angle format and a telephoto format.